Atomic force microscopic observation of SrTiO₃ polar surface

The structure change of SrTiO₃ (111) polar surface was studied as a function of atmosphere. Atomic force microscopy (AFM) and coaxitial impact-collision ion-scattering spectroscopy (CAICISS) were used to study the surface structure of the (111) polar surfaces of SrTiO₃ single crystals. From the result of AFM observation, the polar surfaces annealed in oxygen gas had a trench structure, and that annealed in argon gas formed large triangular terraces. The surface annealed in air, i.e. in intermediate oxygen partial pressure, consisted of complicated self-similar triangles with different sizes, forming a fractal shape called Sierpinski's gasket. The time of flight spectra of CAICISS suggested that a Ti⁴⁺ plane was more stable than a SrO^4-₃ plane on the substrate surface annealed in oxygen gas, and the Ti⁴⁺ and SrO^4-₃ planes almost equally existed on the substrate surface annealed in argon gas. Molecular dynamic calculations at constant temperature and pressure were applied for various SrTiO₃ (111) polar surfaces.